Poorly splits bit of fish substitute (4-2)

I believe the answer is:

fill-in

'substitute' is the definition.
(synonyms)

'poorly splits bit of fish' is the wordplay.
'poorly' becomes 'ill' (similar in meaning).
'splits' means one lot of letters goes inside another (inserted letters split open the word).
'bit of fish' becomes 'fin' (fish have fins).
'ill' put within 'fin' is 'FILL-IN'.

I've seen this clue in The Times.

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